1. Field of the Invention
The present invention relates to lithographic plate imaging, and more particularly to an infrared sensitive photosensitive composition being used to preparation of a lithographic plate which is processed by chemical free developer and a method of preparation of a lithographic plate utilizing the photosensitive composition thereof.
2. Description of Related Arts
The conventional method of preparation of lithography plate is commonly known in the printing industry. There are at least two steps in the process of making the lithographic plate. First, a plate with light sensitive composition is exposed to a particular light source through a mask (such as a positive or a negative mask film) through which a latent image is formed. Second, a subsequent development of the plate after exposure is processed through which excessive coating is removed. A presensitized lithographic plate is a plate which utilizes aluminum or polyester as the support and is prepared through the above two steps to obtain a surface which has both lipophilic and hydrophilic properties for lithographic printing. In general, in a negative-type system, because of the occurrence of polymerization or cross-linking of the coating in the portion under exposure, it becomes insoluble or very difficult to dissolve in the developer and therefore the coating in the portion which is not exposed can be removed during the process of development. Conversely, in a positive-type system, the material in the exposed portion of the plate is removed during the process of development. In general, the process of development includes the step of washing and developing with developer in a treatment unit with developer is used for development. The developer used for positive lithographic plate is a strong base while the developer used for negative lithographic plate is a mixture of strong base and organic solvent such as benzyl alcohol. It is noted that heating method or other methods are also used for image development. The drawback of the above mentioned development process (that is, wet and heat) is time consuming and high cost. In addition, when volatile organic solvent or strong alkaline is used as the developer, treatment of these liquid waste will cause environment problems.
With the development of digital printing, the technological development of plate material, namely CTP, also developed at a fast pace. This technology utilizes software programming to control an on/off status of a laser probe of a CTP platesetter device, then the image to be printed in digitalized format is sent to a presensitized lithographic plate directly and is prepared into the lithographic plate for printing through the above ‘development process’. CTP technology realizes the digitalization of flat plate preparation and eliminates the need of preparation of master film. Admittedly, CTP is a breakthrough in lithography. However, ‘development process’ is still required in this CTP plate preparation and therefore there is still production of liquid waste which causes environmental problems.
In view of the sewage and environmental issues, the researchers in this industry has proposed a variety of solutions which are basically summarized as the three approaches as follows:
The first approach makes use of the phase change technology through sensitive coating to change the lipophilic/hydrophilic properties of the presensitized surface which is then installed onto the printing device directly. In this approach, the lipophilic and hydrophilic properties are solely realized by the light sensitive coating and no excessive coating is removed from the plate such that no pollutants is released from the process of plate preparation through printing and therefore the object of environmental protection is achieved. However, the drawbacks are high energy requirement for plate preparation, instability of ink balance and low run length. Therefore the result is not satisfactory.
Another approach is a type of ‘processed in the machine’ of which the principle is: the presensitized plate is scanned through CTP plate device in which excessive coating in the blank portion for printing is removed through the effect of dampening water and ink and is carried away by the printing paper. The blank portion is hydrophilic aluminum support and this approach achieves the object of zero pollutant emission for environmental protection. However, in view of the published patent application paper, the drawbacks includes dirty on the blank portion, poor run length or ink unbalance difficult to control during the printing process.
Another approach is after the presensitized plate is scanned through CTP plate device, a particular type of ‘non-base or non-solvent’ solvent is used to pretreat the plate to remove the excessive coating in the blank portion before installing onto the printing device for printing. This approach is similar to ‘wet development’ process but the ‘pretreatment solution’ does not have chemicals of base composition or solvent when compared to the developer and the principle of removing the excessive coating in the blank portion is different. The first is a physical process involving dissolution and diffusion while the second is a chemical reaction involving dissolution and diffusion. Therefore, this method is also referred to as ‘non-chemical treatment’ technology. The advantage of this approach is that a more environmental friendly discharge is resulted while a dot correction, which is applicable in conventional CTP plate, is also possible before installing onto the printing device for printing such that getting bad printing plate is avoided. However, this approach also has some drawbacks as follows: the pretreatment solution is not pure water but water which includes chemicals such as dampening agent, diffusing agent or gum Arabic. Therefore, this approach is not truly environmental friendly.